The Human Heart

Introduction

In our world organisms occupy a sliding scale of complexity. On one hand we have the single cell organisms, where all necessary functions for their life are carried out within that one cell. At the other extreme we have extremely complex multicellular organisms, of which humans are perhaps the cardinal member. Obviously, with increased capacity comes increased abilities. Complex organisms are able to manipulate their environment to a greater extent then their simpler cousins. While this has a lot of advantages, it also presents interesting biological problems. With the increased complexity multicellular organisms must have systems to deliver nutrients, signaling molecules, and biochemical building blocks to every cell. In addition waste and useful cellular products need to be removed from the cell and taken to appropriate locations within the organism. This function is accomplished by the circulatory system.

While a variety of organisms have circulatory systems, they varying substantially from primitive organisms to more complex mammals. We in this case are particularly interested in the human circulatory system. At the center of this system lies the heart. The heart is a muscular organ that lies centrally within the thorax. It is suspended by it’s attachment to the great vessels. The heart and attachments are enclosed in a fibrous sac called the pericardium.

Figure

The human heart consists of four chambers, these being left and right atria along with left and right ventricles. The right side of the heart receives blood into the right atrium from the vena cava which is the final venous collection for returning systemic flow. Flowing through the right atrioventricular valve into the right ventricle the blood is th...

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...zations, such as digitalis toxicity, catacholamines, and ischemia. Length of the action potential is also relevant as increased action potential length leads to increased calcium overload. Early afterdepolarizations take place during the plateau (phase 2) and repolarization (phase 3) phases of the cardiac action potential. Like delayed afterdepolarization, early afterdepolarization relies on a prolonged action potential trigger additional activity. The amplitude of the early afterdepolarization is highly dependent on rate. The classical example is a patient with long QT syndrome and bradycardia that is triggered into torsades de pointes.

Reentry is the final category of arrhythmias. Normally, a cardiac action potential is terminated when all the cells have been stimulated and are in refractory. However if a group of cells are somehow able to recover excitability.